Powering RFID tags using multiple synthesized-beam RFID readers

ABSTRACT

Synthesized-beam RFID readers may be used to manage and provide information about RFID tag populations. In one embodiment, two or more synthesized-beam readers synthesize respective RF beams towards a tag location. The synthesized-beam readers may coordinate their pointing by means of a controller, a peer-to-peer network, or by using a master-slave arrangement. The synthesized-beam readers may coordinate their transmissions to increase the RF energy available to a tag at the pointing location.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of a co-pendingU.S. patent application Ser. No. 14/388,793 filed on Sep. 26, 2014,which is a national phase application of International Application no.PCT/US14/26319 filed on Mar. 13, 2014 which claims priority to U.S.Provisional Patent Application Ser. No. 61/784,035 filed on Mar. 14,2013 and 61/887,238 filed on Oct. 4, 2013. The disclosures of theseprovisional patent applications are hereby incorporated by reference forall purposes.

BACKGROUND

Radio-Frequency Identification (RFID) systems typically include RFIDreaders, also known as RFID reader/writers or RFID interrogators, andRFID tags. RFID systems can be used in many ways for locating andidentifying objects to which the tags are attached. RFID systems areuseful in product-related and service-related industries for trackingobjects being processed, inventoried, or handled. In such cases, an RFIDtag is usually attached to an individual item, or to its package.

In principle. RFID techniques entail using an RFID reader to inventoryone or more RFID tags, where inventorying involves at least singulatinga tag and receiving an identifier from the singulated tag (where“singulated” is defined as an individual tag singled-out by a reader,potentially from among multiple tags, and an “identifier” is defined asany number identifying the tag or the item to which the tag is attached,such as a tag identifier (TID), an electronic product code (EPC), etc.).The reader transmitting a Radio Frequency (RF) wave performs theinterrogation. The RF wave is typically electromagnetic, at least in thefar field. The RF wave can also be predominantly electric or magnetic inthe near field. The RF wave may encode one or more commands thatinstruct the tags to perform one or more actions. In typical RFIDsystems, an RFID reader transmits a modulated RF inventory signal (acommand), receives a tag reply, and transmits an RF acknowledgementsignal responsive to the tag reply.

A tag that senses the interrogating RF wave may respond by transmittingback another RF wave. The tag either generates the transmitted back RFwave originally, or by reflecting back a portion of the interrogating RFwave in a process known as backscatter. Backscatter may take place in anumber of ways.

The reflected-back RF wave may encode data stored in the tag, such as anumber. The response is demodulated and decoded by the reader, whichthereby identifies, counts, or otherwise interacts with the associateditem. The decoded data can denote a serial number, a price, a date, adestination, other attribute(s), any combination of attributes, and soon. Accordingly, when a reader receives tag data it can learn about theitem that hosts the tag and/or about the tag itself.

An RFID tag typically includes an antenna section, a radio section, apower-management section, and frequently a logical section, a memory, orboth. In some RFID tags the power-management section included an energystorage device such as a battery. RFID tags with an energy storagedevice are known as battery-assisted, semi-active, or active tags. OtherRFID tags can be powered solely by the RF signal they receive. Such RFIDtags do not include an energy storage device and are called passivetags. Of course, even passive tags typically include temporary energy-and data/flag-storage elements such as capacitors or inductors.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended asan aid in determining the scope of the claimed subject matter.

Embodiments are directed to powering RFID tags using multiplesynthesized-beam RFID readers. A synthesized-beam RFID reader, whichcomprises at least one RFID reader and an antenna array, electricallysynthesizes multiple beam patterns by adjusting the signals provided tothe antenna elements of the array. The multiple beam patterns may pointin different physical directions, may provide different beam shapes, mayprovide different physical coverage, or a may provide mix of theseattributes. The reader may comprise single or multiple transmitters,single or multiple receivers, be separate from and connected to elementsof the antenna array, or be distributed and embedded within the elementsof the array. Either the reader or an array controller may adjust thephase and/or amplitude of the signals provided to the array elements tosynthesize the multiple beams. The antenna array may comprise multiplediscrete antenna elements or may employ a continuous structure that canemulate multiple antennas. By switching among the beams, asynthesized-beam reader may scan its environment, essentially steeringits gaze in different directions and with potentially different beamshapes as it scans. As a simple but not-limiting example of asynthesized-beam system, consider the antenna array on a U.S. Navy shipthat forms a synthesized-beam radar, and envision the radar scanning theenvironment to inventory RFID tags rather than scanning the environmentto detect distant ships or airplanes. Like a synthesized-beam radar, asynthesized-beam RFID reader may use multiple RF frequencies, differentbeam shapes, different beam directions, and different signal waveshapesto inventory/locate/track its target tags.

Embodiments are directed to interrogating (defined as inventoryingand/or accessing) an RFID tag using multiple synthesized-beam RFIDreaders. A first synthesized-beam reader synthesizes a first beam toinventory a tag at a certain physical location. A secondsynthesized-beam reader simultaneously synthesizes a second beam towardsthe same physical location to “boost” or otherwise improve thelikelihood or the performance of the interrogation. Unlike radar systemsinterrogating remote objects, RFID tags extract power from theinterrogating wave and modulate their antenna reflectance to generate abackscattered signal. Even more unlike radar systems, some RFID tags areable to extract power from a wave transmitted by one reader even as theyare responding to an interrogating signal in the wave transmitted byanother. In these embodiments, the first reader will transmit commandsto, and receive responses from, the tag, while the second will transmitan unmodulated or minimally modulated wave to the tag to boost the tag'sextracted power and thereby extend its interrogation range.

Of course, those skilled in the art will recognize many possiblevariants on the above-described two-reader scenario. As one example, thefirst reader may transmit commands, the second reader may transmitpower, and the second reader (rather than the first) may receive thetag's responses. As another example, the first reader may transmitcommands to, and receive responses from, the tag, while second and third(or perhaps even more) RFID readers deliver power to the tag. As yetanother example, both first and second RFID readers may transmit thesame command to the tag, thereby delivering both a stronger commandsignal and more power to the tag.

Embodiments are directed to methods of directing at least twosynthesized-beam RFID readers to point to a common physical location toinventory an RFID tag. Such methods may include one synthesized-beamreader acting as a master and directing other readers to point to thelocation, peer-to-peer communications among synthesized-beam RFIDreaders to indicate the location, or a controller directing thesynthesized-beam RFID readers to point to the location. The directingmay include choosing the physical location, the beam shape, the durationof the interrogation, the transmit frequencies of the varioussynthesized-beam RFID readers, which reader(s) is/are sending commandsand which are sending RF power, the commands to be transmitted, commandparameters (such as datarate, modulation format, reply frequency, andother communication parameters as will be well known to those skilled inthe art), which readers are receiving the tag response, the polarizationof the transmit and receive beams, the transmit power, and the receivesensitivity.

Embodiments are also directed to techniques for tracking an RFID tagusing synthesized-beam RFID readers, including using flag refreshcommands to segment RFID tag subpopulations and thereby evaluate orpredict tag movement within a background of stationary tags.

These and other features and advantages will be apparent from a readingof the following detailed description and a review of the associateddrawings. It is to be understood that both the foregoing generaldescription and the following detailed description are explanatory onlyand are not restrictive of aspects as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following Detailed Description proceeds with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of components of an RFID system.

FIG. 2 is a diagram showing components of a passive RFID tag, such as atag that can be used in the system of FIG. 1.

FIG. 3 is a conceptual diagram for explaining a half-duplex mode ofcommunication between the components of the RFID system of FIG. 1.

FIG. 4 is a block diagram showing a detail of an RFID IC.

FIGS. 5A and 5B illustrate signal paths during tag-to-reader andreader-to-tag communications in the block diagram of FIG. 4.

FIG. 6 is a block diagram of a whole RFID reader system according toembodiments.

FIG. 7 is a block diagram illustrating an overall architecture of anRFID system according to embodiments.

FIG. 8 depicts a discrete-element antenna array according toembodiments.

FIGS. 9A and 9B depict the antenna array of FIG. 8 synthesizing a beamin different physical directions, according to embodiments.

FIG. 10 depicts some of the potential beam locations that can besynthesized by the antenna array of FIG. 8, according to embodiments.

FIG. 11 depicts radiated beam power as a function of beam angle for asubset of the potential beams of the antenna array of FIG. 8.

FIG. 12 depicts an RFID tag located in the subset of the potential beamsof FIG. 11.

FIG. 13 depicts how the location of the RFID tag of FIG. 12 can bedetermined using tag sensitivity and radiated beam power.

FIG. 14 depicts how a tag's location may be determined using multiplebeams.

FIG. 15 depicts beams with non-circular beam shapes formed by asynthesized-beam reader according to embodiments.

FIG. 16 depicts beams with sidelobes formed by a synthesized-beam readeraccording to embodiments.

FIG. 17 depicts how frequency-based variations in beam power can be usedto determine tag location.

FIG. 18 depicts a process for determining tag location by counting tagreads of a synthesized-beam reader.

FIG. 19 depicts the effective tag inventory range of a synthesized-beamreader according to embodiments.

FIG. 20 depicts how a synthesized-beam reader's effective tag inventoryrange can be increased using another synthesized-beam reader accordingto embodiments.

FIG. 21 depicts how multiple synthesized-beam readers can cooperate tocommunicate with a population of tags according to embodiments.

FIG. 22 depicts methods of controlling multiple synthesized-beam readersaccording to embodiments.

FIG. 23 depicts a process for using cooperating synthesized-beam readersto enhance tag inventory range according to embodiments.

FIG. 24 is a diagram showing the effects of a broadcast refresh on tagflag physical parameters as a function of time, according toembodiments;

FIG. 25 is a conceptual diagram showing an illustrative inventoryingprocess without broadcast refresh according to embodiments;

FIG. 26 is a conceptual diagram showing an illustrative inventoryingprocess with broadcast refresh according to embodiments;

FIG. 27 is a conceptual diagram showing side views of a synthesized-beamreader at different stages of a tag motion-tracking process according toembodiments.

FIG. 28 depicts a timing diagram for a tag tracking process with tagrefresh commands according to embodiments.

FIG. 29 is a flowchart of a tag tracking process according toembodiments.

DETAILED DESCRIPTION

In the following detailed description, references are made to theaccompanying drawings that form a part hereof, and in which are shown byway of illustration specific embodiments or examples. These embodimentsor examples may be combined, other aspects may be utilized, andstructural changes may be made without departing from the spirit orscope of the present disclosure. The following detailed description istherefore not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims and theirequivalents.

As used herein, “memory” is one of ROM, RAM, SRAM, DRAM, NVM, EEPROM,FLASH, Fuse, MRAM, FRAM, and other similar information-storagetechnologies as will be known to those skilled in the art. Some portionsof memory may be writeable and some not. “Command” refers to a readerrequest for one or more tags to perform one or more actions, andincludes one or more instructions. “Instruction” refers to a request toa tag to perform an action (e.g., write data into memory). “Program”refers to a request to a tag to perform a set or sequence ofinstructions (e.g., read a value from memory and, if the read value isless than a threshold then lock a memory word). “Protocol” refers to anindustry standard for communications between a reader and a tag (andvice versa), such as the Class-1 Generation-2 UHF RFID Protocol forCommunications at 860 MHz-960 MHz by GS1 EPCglobal, Inc. (“Gen2Specification”), versions 1.2.0 and 2.0.0 of which are herebyincorporated by reference.

FIG. 1 is a diagram of the components of a typical RFID system 100,incorporating embodiments. An RFID reader 110 transmits an interrogatingRF signal 112. RFID tag 120 in the vicinity of RFID reader 110 sensesinterrogating RF signal 112 and generate signal 126 in response. RFIDreader 110 senses and interprets signal 126. The signals 112 and 126 mayinclude RF waves and/or non-propagating RF signals (e.g., reactivenear-field signals)

Reader 110 and tag 120 communicate via signals 112 and 126, which areamplitude- and/or phase-modulated waves. When communicating, eachencodes, modulates, and transmits data to the other, and each receives,demodulates, and decodes data from the other. The data can be modulatedonto, and demodulated from, RF waves, such as for signals 112 and 126.The RF waves are typically in a suitable range of frequencies, such asthose near 900 MHz, 13.56 MHz, and so on.

The communication between reader and tag uses symbols, also called RFIDsymbols. A symbol can be a delimiter, a calibration value, and so on.Symbols can be implemented for exchanging binary data, such as “0” and“1”, if that is desired. When symbols are processed by reader 110 andtag 120 they can be treated as values, numbers, and so on.

Tag 120 can be a passive tag, or an active or battery-assisted tag(i.e., a tag having its own power source). When tag 120 is a passivetag, it is powered from signal 112.

FIG. 2 is a diagram of an RFID tag 220, which may function as tag 120 ofFIG. 1. Tag 220 is drawn as a passive tag, meaning it does not have itsown power source. Much of what is described in this document, however,applies also to active and battery-assisted tags.

Tag 220 is typically (although not necessarily) formed on asubstantially planar inlay 222, which can be made in many ways known inthe art. Tag 220 includes a circuit which may be implemented as an IC224. In some embodiments IC 224 is implemented in complementarymetal-oxide semiconductor (CMOS) technology. In other embodiments IC 224may be implemented in other technologies such as bipolar junctiontransistor (BJT) technology, metal-semiconductor field-effect transistor(MESFET) technology, and others as will be well known to those skilledin the art. IC 224 is arranged on inlay 222.

Tag 220 also includes an antenna for exchanging wireless signals withits environment. The antenna is often flat and attached to inlay 222. IC224 is electrically coupled to the antenna via suitable IC contacts (notshown in FIG. 2). The term “electrically coupled” as used herein meansthat a low-impedance path exists between the electrically coupledcomponents, and may mean the presence of a direct electrical connectionor a connection that includes one or more intervening circuit blocks,elements, or devices. The “electrical” part of the term “electricallycoupled” as used in this document shall mean a coupling that is one ormore of ohmic/galvanic, capacitive, and/or inductive. Similarly, theterm “electrically isolated” as used herein means that electricalcoupling of one or more types (e.g., galvanic, capacitive, and/orinductive) is not present, at least to the extent possible. For example,elements that are electrically isolated from each other are galvanicallyisolated from each other, capacitively isolated from each other, and/orinductively isolated from each other. Of course, electrically isolatedcomponents will generally have some unavoidable stray capacitive orinductive coupling between them, but the intent of the isolation is tominimize this stray coupling to a negligible level when compared with anelectrically coupled path.

IC 224 is shown with a single antenna port, comprising two IC contactselectrically coupled to two antenna segments 226 and 228 which are shownhere forming a dipole. Many other embodiments are possible using anynumber of ports, contacts, antennas, and/or antenna segments.

Diagram 250 depicts top and side views of tag 252, formed using a strap.Tag 252 differs from tag 220 in that it includes a substantially planarstrap substrate 254 having strap contacts 256 and 258. IC 224 is mountedon strap substrate 254 such that the IC contacts on IC 224 electricallycouple to strap contacts 256 and 258 via suitable connections (notshown). Strap substrate 254 is then placed on inlay 222 such that strapcontacts 256 and 258 electrically couple to antenna segments 226 and228. Strap substrate 254 may be affixed to inlay 222 via pressing, aninterface layer, one or more adhesives, or any other suitable means.

Diagram 260 depicts a side view of an alternative way to place strapsubstrate 254 onto inlay 222. Instead of strap substrate 254's surface,including strap contacts 256/258, facing the surface of inlay 222, strapsubstrate 254 is placed with its strap contacts 256/258 facing away fromthe surface of inlay 222. Strap contacts 256/258 can then be eithercapacitively coupled to antenna segments 226/228 through strap substrate254, or conductively coupled using a through-via which may be formed bycrimping strap contacts 256/258 to antenna segments 226/228. In someembodiments the positions of strap substrate 254 and inlay 222 may bereversed, with strap substrate 254 mounted beneath strap substrate 222and strap contacts 256/258 electrically coupled to antenna segments226/228 through inlay 222. Of course, in yet other embodiments strapcontacts 256/258 may electrically couple to antenna segments 226/228through both inlay 222 and strap substrate 254.

In operation, the antenna receives a signal and communicates it to IC224, which both harvests power and responds if appropriate, based on theincoming signal and the IC's internal state. If IC 224 uses backscattermodulation then it responds by modulating the antenna's reflectance,which generates response signal 126 from signal 112 transmitted by thereader. Electrically coupling and uncoupling the antenna contacts of IC224 can modulate the antenna's reflectance, as can varying theadmittance of a shunt-connected circuit element which is coupled to theantenna contacts. Varying the impedance of a series-connected circuitelement is another means of modulating the antenna's reflectance.

In the embodiments of FIG. 2, antenna segments 226 and 228 are separatefrom IC 224. In other embodiments the antenna segments may alternativelybe formed on IC 224. Tag antennas according to embodiments may bedesigned in any form and are not limited to dipoles. For example, thetag antenna may be a patch, a slot, a loop, a coil, a horn, a spiral, amonopole, microstrip, stripline, or any other suitable antenna.

The components of the RFID system of FIG. 1 may communicate with eachother in any number of modes. One such mode is called full duplex.Another such mode is called half-duplex, and is described below.

FIG. 3 is a conceptual diagram 300 for explaining half-duplexcommunications between the components of the RFID system of FIG. 1, inthis case with tag 120 implemented as passive tag 220 of FIG. 2. Theexplanation is made with reference to a TIME axis, and also to a humanmetaphor of “talking” and “listening”. The actual technicalimplementations for “talking” and “listening” are now described.

RFID reader 110 and RFID tag 120 talk and listen to each other by takingturns. As seen on axis TIME, when reader 110 talks to tag 120 thecommunication session is designated as “R→T”, and when tag 120 talks toreader 110 the communication session is designated as “T→R”. Along theTIME axis, a sample R→T communication session occurs during a timeinterval 312, and a following sample T→R communication session occursduring a time interval 326. Of course interval 312 is typically of adifferent duration than interval 326—here the durations are shownapproximately equal only for purposes of illustration.

According to blocks 332 and 336, RFID reader 110 talks during interval312, and listens during interval 326. According to blocks 342 and 346,RFID tag 120 listens while reader 110 talks (during interval 312), andtalks while reader 110 listens (during interval 326).

In terms of actual behavior, during interval 312 reader 110 talks to tag120 as follows. According to block 352, reader 110 transmits signal 112,which is a modulated RF signal as described in FIG. 1. At the same time,according to block 362, tag 120 receives signal 112 and processes it toextract data and so on. Meanwhile, according to block 372, tag 120 doesnot backscatter with its antenna, and according to block 382, reader 110has no signal to receive from tag 120.

During interval 326, tag 120 talks to reader 110 as follows. Accordingto block 356, reader 110 transmits a Continuous Wave (CW) signal, whichcan be thought of as a carrier RF signal that is typically not amplitudemodulated or phase modulated and therefore encodes no information. ThisCW signal serves both to transfer energy to tag 120 for its own internalpower needs, and also as a carrier that tag 120 can modulate with itsbackscatter. Indeed, during interval 326, according to block 366, tag120 does not receive a signal for processing. Instead, according toblock 376, tag 120 modulates the CW emitted according to block 356 so asto generate backscatter signal 126. Concurrently, according to block386, reader 110 receives backscatter signal 126 and processes it.

FIG. 4 is a block diagram showing a detail of an RFID IC, such as IC 224in FIG. 2. Electrical circuit 424 in FIG. 4 may be formed in an IC of anRFID tag, such as tag 220 of FIG. 2. Circuit 424 has a number of maincomponents that are described in this document. Circuit 424 may have anumber of additional components from what is shown and described, ordifferent components, depending on the exact implementation.

Circuit 424 shows two IC contacts 432 and 433 suitable for coupling toantenna segments such as segments 226/228 of RFID tag 220 of FIG. 2.When two IC contacts form the signal input from and signal return to anantenna they are often referred-to as an antenna port. IC contacts 432and 433 may be made in any suitable way, such as with metallic pads andso on. In some embodiments circuit 424 uses more than two contacts,especially when tag 220 has more than one antenna port and/or more thanone antenna.

Circuit 424 includes signal-routing section 435 which may include signalwiring, signal-routing busses, receive/transmit switches that canselectively route a signal, and so on. In some embodiments, circuit 424includes optional capacitors 436 and/or 438. If present, capacitors436/438 capacitively couple IC contacts 432/433 to signal-routingsection 435, which in turn electrically couples to other components ofcircuit 424 described below. This capacitive coupling causes IC contacts432/433 to be galvanically decoupled from signal-routing section 435 andother circuit components.

Capacitive coupling (and resultant galvanic decoupling) between ICcontacts 432 and/or 433 and components of circuit 424 is desirable incertain situations. For example, in some RFID tag embodiments ICcontacts 432 and 433 may galvanically connect to terminals of a tuningloop on the tag. In this situation, capacitors 436 and/or 438galvanically decouple IC contact 432 from IC contact 433, therebypreventing the formation of a short circuit between the IC contactsthrough the tuning loop.

Capacitors 436/438 may be implemented within circuit 424 and/or at leastpartly external to circuit 424. For example, a dielectric or insulatinglayer on the surface of the IC containing circuit 424 may serve as thedielectric in capacitor 436 and/or capacitor 438. As another example, adielectric or insulating layer on the surface of a tag substrate (e.g.,inlay 222 or strap substrate 254) may serve as the dielectric incapacitors 436/438. Metallic or conductive layers positioned on bothsides of the dielectric layer (i.e., between the dielectric layer andthe IC and between the dielectric layer and the tag substrate) may thenserve as terminals of the capacitors 436/438. The conductive layers mayinclude IC contacts (e.g., IC contacts 432/433), antenna segments (e.g.,antenna segments 226/228), or any other suitable conductive layers.

Circuit 424 also includes a rectifier and PMU (Power Management Unit)441 that harvests energy from the RF signal received by the antenna topower the circuits of IC 424 during either or both reader-to-tag (R→T)and tag-to-reader (T→R) sessions. Rectifier and PMU 441 may beimplemented in any way known in the art.

Circuit 424 additionally includes a demodulator 442 that demodulates theRF signal received via IC contacts 432, 433. Demodulator 442 may beimplemented in any way known in the art, for example including a slicer,an amplifier, and so on.

Circuit 424 further includes a processing block 444 that receives theoutput from demodulator 442 and performs operations such as commanddecoding, memory interfacing, and so on. In addition, processing block444 may generate an output signal for transmission. Processing block 444may be implemented in any way known in the art, for example bycombinations of one or more of a processor, memory, decoder, encoder,and so on.

Circuit 424 additionally includes a modulator 446 that modulates anoutput signal generated by processing block 444. The modulated signal istransmitted by driving IC contacts 432, 433, and therefore driving theload presented by the coupled antenna segment or segments. Modulator 446may be implemented in any way known in the art, for example including aswitch, driver, amplifier, and so on.

In one embodiment, demodulator 442 and modulator 446 may be combined ina single transceiver circuit. In another embodiment modulator 446 maymodulate a signal using backscatter. In another embodiment modulator 446may include an active transmitter. In yet other embodiments demodulator442 and modulator 446 may be part of processing block 444.

Circuit 424 additionally includes a memory 450 to store data 452. Atleast a portion of memory 450 is preferably implemented as a NonvolatileMemory (NVM), which means that data 452 is retained even when circuit424 does not have power, as is frequently the case for a passive RFIDtag.

In some embodiments, particularly in those with more than one antennaport, circuit 424 may contain multiple demodulators, rectifiers, PMUs,modulators, processing blocks, and/or memories.

In terms of processing a signal, circuit 424 operates differently duringa R→T session and a T→R session. The different operations are describedbelow, in this case with circuit 424 representing an IC of an RFID tag.

FIG. 5A shows version 524-A of components of circuit 424 of FIG. 4,further modified to emphasize a signal operation during a R→T sessionduring time interval 312 of FIG. 3. Demodulator 442 demodulates an RFsignal received from IC contacts 432, 433. The demodulated signal isprovided to processing block 444 as C_IN. In one embodiment, C_IN mayinclude a received stream of symbols.

Version 524-A shows as relatively obscured those components that do notplay a part in processing a signal during a R→T session. Rectifier andPMU 441 may be active, such as for converting RF power. Modulator 446generally does not transmit during a R→T session, and typically does notinteract with the received RF signal significantly, either becauseswitching action in section 435 of FIG. 4 decouples modulator 446 fromthe RF signal, or by designing modulator 446 to have a suitableimpedance, and so on.

Although modulator 446 is typically inactive during a R→T session, itneed not be so. For example, during a R→T session modulator 446 could beadjusting its own parameters for operation in a future session, and soon.

FIG. 5B shows version 524-B of components of circuit 424 of FIG. 4,further modified to emphasize a signal operation during a T→R sessionduring time interval 326 of FIG. 3. Processing block 444 outputs asignal C_OUT. In one embodiment, C_OUT may include a stream of symbolsfor transmission. Modulator 446 then modulates C_OUT and provides it toantenna segments such as segments 226/228 of RFID tag 220 via ICcontacts 432, 433.

Version 524-B shows as relatively obscured those components that do notplay a part in processing a signal during a T→R session. Rectifier andPMU 441 may be active, such as for converting RF power. Demodulator 442generally does not receive during a T→R session, and typically does notinteract with the transmitted RF signal significantly, either becauseswitching action in section 435 of FIG. 4 decouples demodulator 442 fromthe RF signal, or by designing demodulator 442 to have a suitableimpedance, and so on.

Although demodulator 442 is typically inactive during a T→R session, itneed not be so. For example, during a T→R session demodulator 442 couldbe adjusting its own parameters for operation in a future session, andso on.

In typical embodiments, demodulator 442 and modulator 446 are operableto demodulate and modulate signals according to a protocol. A protocolis a specification or industry standard such as the Gen2 Specificationdescribed above that calls for specific manners of signaling between thereader and the tags. A protocol specifies, in part, symbol encodings,and may include a set of modulations, rates, timings, or any otherparameter associated with data communications.

In addition, a protocol can be a variant of a stated specification suchas the Gen2 Specification, for example including fewer or additionalcommands than the stated specification calls for, and so on. In suchinstances, additional commands are sometimes called custom commands. Inembodiments where circuit 424 includes multiple demodulators and/ormodulators, each may be configured to support different protocols ordifferent sets of protocols.

FIG. 6 is a block diagram of a whole RFID reader system 600 according toembodiments. System 600 includes a local block 610, and optionallyremote components 670. Local block 610 and remote components 670 can beimplemented in any number of ways. It will be recognized that reader 110of FIG. 1 is the same as local block 610, if remote components 670 arenot provided. Alternately, reader 110 can be implemented instead bysystem 600, of which only the local block 610 is shown in FIG. 1.

Local block 610 is responsible for communicating with tags. Local block610 includes a block 651 of an antenna and a driver of the antenna forcommunicating with the tags. Some readers, like that shown in localblock 610, contain a single antenna and driver. Some readers containmultiple antennas and drivers and a method to switch signals among them,including sometimes using different antennas for transmitting and forreceiving. And some readers contain multiple antennas and drivers thatcan operate simultaneously. A demodulator/decoder block 653 demodulatesand decodes backscattered waves received from the tags via antenna block651. Modulator/encoder block 654 encodes and modulates RF waves that areto be transmitted to the tags via antenna/driver block 651.

Local block 610 additionally includes an optional local processor 656.Local processor 656 may be implemented in any number of ways known inthe art. Such ways include, by way of examples and not of limitation,digital and/or analog processors such as microprocessors anddigital-signal processors (DSPs); controllers such as microcontrollers;software running in a machine such as a general purpose computer,programmable circuits such as Field Programmable Gate Arrays (FPGAs),Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices(PLDs), Application Specific Integrated Circuits (ASIC), any combinationof one or more of these; and so on. In some cases some or all of thedecoding function in block 653, the encoding function in block 654, orboth, may be performed instead by local processor 656. In some caseslocal processor 656 may implement an encryption or authorizationfunction; in some cases one or more of these functions can bedistributed among other blocks such as encoding block 654, or may beentirely incorporated in another block.

Local block 610 additionally includes an optional local memory 657.Local memory 457 may be implemented in any number of ways known in theart, including, by way of example and not of limitation, any of thememory types described above as well as any combination thereof. Localmemory 657 can be implemented separately from local processor 656, or inan IC with local processor 656, with or without other components. Localmemory 657, if provided, can store programs for local processor 656 torun, if needed.

In some embodiments, local memory 657 stores data read from tags, ordata to be written to tags, such as Electronic Product Codes (EPCs), TagIdentifiers (TIDs), keys, hashes, and other data. Local memory 657 canalso include reference data that is to be compared to the EPC,instructions and/or rules for how to encode commands for the tags, modesfor controlling antenna 651, and so on. In some of these embodiments,local memory 657 is provided as a database.

Some components of local block 610 typically treat the data as analog,such as the antenna/driver block 651. Other components such as localmemory 657 typically treat the data as digital. At some point there is aconversion between analog and digital. Based on where this conversionoccurs, a whole reader may be characterized as “analog” or “digital”,but most readers contain a mix of analog and digital functionality.

If remote components 670 are indeed provided, they are coupled to localblock 610 via an electronic communications network 680. Network 680 canbe a Local Area Network (LAN), a Metropolitan Area Network (MAN), a WideArea Network (WAN), a network of networks such as the internet, or amere local communication link, such as a USB, PCI, and so on. Localblock 610 may include a local network connection 659 for communicatingwith network 680. Communications on the network can be secure, such asif they are encrypted or physically protected, or insecure if they arenot encrypted or otherwise protected.

There can be one or more remote component(s) 670. If more than one, theycan be located at the same location, or in different locations. They canaccess each other and local block 610 via communications network 680, orvia other similar networks, and so on. Accordingly, remote component(s)670 can use respective remote network connections. Only one such remotenetwork connection 679 is shown, which is similar to local networkconnection 659, etc.

Remote component(s) 670 can also include a remote processor 676. Remoteprocessor 676 can be made in any way known in the art, such as wasdescribed with reference to local processor 656.

Remote component(s) 670 can also include a remote memory 677. Remotememory 677 can be made in any way known in the art, such as wasdescribed with reference to local memory 657. Remote memory 677 mayinclude a local database, and a different database of a StandardsOrganization, such as one that can reference EPCs. Remote memory 677 mayalso contain information associated with commands, tag profiles, keys,or the like, similar to local memory 657

Of the above-described elements, it is advantageous to consider acombination of these components, designated as operational processingblock 690. Operational processing block 690 includes those componentsthat are provided of the following: local processor 656, remoteprocessor 676, local network connection 659, remote network connection679, and by extension an applicable portion of communications network680 that links remote network connection 659 with local networkconnection 679. The portion can be dynamically changeable, etc. Inaddition, operational processing block 690 can receive and decode RFwaves received via antenna driver 651, and cause antenna driver 651 totransmit RF waves according to what it has processed.

Operational processing block 690 includes either local processor 656, orremote processor 676, or both. If both are provided, remote processor676 can be made such that it operates in a way complementary with thatof local processor 656. In fact, the two can cooperate. It will beappreciated that operational processing block 690, as defined this way,is in communication with both local memory 657 and remote memory 677, ifboth are present.

Accordingly, operational processing block 690 is location agnostic, inthat its functions can be implemented either by local processor 656, byremote processor 676, or by a combination of both. Some of thesefunctions are preferably implemented by local processor 656, and some byremote processor 676. Operational processing block 690 accesses localmemory 657, or remote memory 677, or both for storing and/or retrievingdata.

RFID reader system 600 operates by operational processing block 690generating communications for RFID tags. These communications areultimately transmitted by antenna driver block 651, withmodulator/encoder block 654 encoding and modulating the information onan RF wave. Then data is received from the tags via antenna driver block651, demodulated and decoded by demodulator/decoder block 653, andprocessed by operational processing block 690.

Embodiments of an RFID reader system can be implemented as hardware,software, firmware, or any combination. It is advantageous to considersuch a system as subdivided into components or modules. A person skilledin the art will recognize that some of these components or modules canbe implemented as hardware, some as software, some as firmware, and someas a combination. An example of such a subdivision is now described,together with the RFID tag as an additional module.

FIG. 7 is a block diagram illustrating an architecture of an RFID system700 according to embodiments. For reasons of clarity, RFID system 700 issubdivided into modules or components. Each of these modules may beimplemented by itself, or in combination with others. In addition, someof them may be present more than once. Other embodiments may beequivalently subdivided into different modules. It will be recognizedthat some aspects of FIG. 7 are parallel with those describedpreviously.

RFID tag 703 is considered here as a module by itself. RFID tag 703conducts a wireless communication 706 with the remainder, via the airinterface 705. Air interface 705 is really a boundary, in that signalsor data that pass through it are not intended to be transformed from onething to another. Specifications as to how readers and tags are tocommunicate with each other, for example the Gen2 Specification, alsoproperly characterize that interface as a boundary.

RFID system 700 includes one or more reader antennas 710, and an RFfront-end module 720 for interfacing with reader antenna(s) 710. Thesecan be made as described above.

RFID system 700 also includes a signal-processing module 730. In oneembodiment, signal-processing module 730 exchanges waveforms with RFfront-end module 720, such as I and Q waveform pairs.

RFID system 700 also includes a physical-driver module 740, which isalso known as data-link module. In some embodiments physical-drivermodule 740 exchanges bits with signal-processing module 730.Physical-driver module 740 can be the stage associated with the framingof data.

RFID system 700 additionally includes a media access control module 750,which is also known as MAC layer module. In one embodiment, MAC layermodule 750 exchanges packets of bits with physical driver module 740.MAC layer module 750 can make decisions for sharing the medium ofwireless communication, which in this case is the air interface but inother embodiments could be a wired interface.

RFID system 700 moreover includes an application-programminglibrary-module 760, which can include application programming interfaces(APIs), other objects, etc.

All of these RFID system functionalities can be supported by one or moreprocessors. One of these processors can be considered a host processor.Such a host processor might include a host operating system (OS) and/orcentral processing unit (CPU), as in module 770. In some embodiments,the processor is not considered as a separate module, but one thatincludes some of the above-mentioned modules of RFID system 700.

User interface module 780 may be coupled toapplication-programming-library module 760, for accessing the APIs. Userinterface module 780 can be manual, automatic, or both. It can besupported by the host OS/CPU module 770 mentioned above, or by aseparate processor, etc.

It will be observed that the modules of RFID system 700 form a chain.Adjacent modules in the chain can be coupled by appropriateinstrumentalities for exchanging signals. These instrumentalitiesinclude conductors, buses, interfaces, and so on. Theseinstrumentalities can be local, e.g. to connect modules that arephysically close to each other, or over a network, for remotecommunication.

The chain is used in one direction for receiving RFID waveforms and inthe other for transmitting RFID waveforms. In receiving mode, readerantenna(s) 710 receives wireless waves, which are in turn processedsuccessively by the various modules in the chain. Processing canterminate in any one of the modules. In transmitting mode, waveforminitiation can be in any one of the modules. Ultimately, signals arerouted to reader antenna(s) 710 to be transmitted as wireless waves.

The architecture of RFID system 700 is presented for purposes ofexplanation, and not of limitation. Its particular, subdivision intomodules need not be followed for creating embodiments. Furthermore, thefeatures of the present disclosure can be performed either within asingle one of the modules, or by a combination of them.

Everything described above in terms of readers and reader componentsfinds some correspondence with tags and tag ICs, and vice versa.Numerous details have been set forth in this description, which is to betaken as a whole, to provide a more thorough understanding of theinvention. In other instances, well-known features have not beendescribed in detail, so as to not obscure unnecessarily the invention.

One or more RFID readers, or distributed portions of one or morereaders, may be coupled to or embedded within an antenna array to form asynthesized-beam reader (SBR) capable of generating multiple RF beams,as described above. FIG. 8 depicts a perspective view of an antennaarray 800 with discrete radiating elements according to embodiments.Antenna array 800 includes an array of antenna elements 802 and 804, anda ground plane 808 behind elements 802 and 804. Each element has aradiating direction vector 806 (only shown for one element) that istypically, but not necessarily, perpendicular to the ground plane. An RFradiation pattern (or “beam”) for receiving or transmitting an RF signalmay be synthesized by adjusting the amplitude and/or phase of thesignals coupled from/to each antenna element 802 and 804. The directionof the synthesized beam (typically represented by the direction of thebeam's primary lobe—the lobe having the highest radiated power) iscontrolled by these various amplitude and/or phase adjustments. Theadjustments may be analog, digital, or a mix of analog and digital. Forexample, during transmission, an SBR may generate the signal to betransmitted and then direct the generated signal to elements 802 and 804with different amplitudes and phases. Alternatively, the SBR maysynthesize the different signals for each antenna element digitally andthen convert the digital signals to analog. In other embodiments the SBRmay use a mix of these approaches. Similarly, during a receive operationthe SBR may combine analog signals after appropriate phase shifting andamplitude adjustment of each, or it may digitize the signals from eachelement and combine them digitally, or a mix thereof.

The antenna elements of SBA 800 may be one or more of patch, slot, wire,horn, helical, distributed, or any other type as will be known to thoseskilled in the art. Whereas FIG. 8 only shows nine antenna elements,antenna arrays with any number of antenna elements may be used,including a single distributed element or an element made frommetamaterials. In some embodiments ground plane 808 may be nonplanar(e.g., curved, concave, convex, etc.) and in other embodiments need notexist.

FIGS. 9A and 9B show the directions of some of the RF beams that SBA900, similar to SBA 800 in FIG. 8, can generate. SBA 900 has nineantenna elements 902-918, with element 902 at the center and elements904-918 around it. The shape and direction of the beam that SBA 900generates depends on the signals to/from each element. Suppose that SBR900 transmits using primarily elements 902, 906, and 914. Then,depending on the amplitude and phase of the signals applied to theseelements, SBA 900 can steer a beam along the direction indicated bydashed line 920. In a similar fashion, suppose that SBR 900 transmitsprimarily using elements 902, 908, and 916. Then, depending on theamplitude and phase of the signals applied to these elements, SBA 900can steer a beam along the direction indicated by dashed line 922. Ofcourse, other steering arrangements are possible, including using all 9elements to transmit and/or receive in arbitrary directions and togenerate narrow beams.

FIG. 9B shows how RF beams with different directions can be synthesizedusing antenna elements located along line 920, with the diagram to theleft depicting a head-on view similar to FIG. 9A and the diagram to theright depicting a side view. As described above, the beam direction canbe controlled by varying the amplitude and phase of the signals to/fromthe antenna elements. For example, by applying a leading signal phase toelement 906, an intermediate signal phase to element 902, and a trailingsignal phase to element 914, the SBA will tend to steer its beamdownward as in beam 934. Switching leading and lagging from elements906/902 to elements 902/906 will tend to steer the beam upwards as inbeam 930. Of course, the actual beam shape depends on both the magnitudeof the phase shifting and the magnitude of the amplitude scaling (ifany).

FIG. 10 depicts potential beams from an SBR according to embodiments.Diagram 1000 depicts a side perspective of SBR 1010, capable ofsynthesizing at least five different RF beams 1012, 1013, 1014, 1015,and 1016, arranged along line 1018 (similar to line 920 in FIG. 9A),with each RF beam pointed in a different direction.

Diagrams 1020, 1040, 1060, and 1080 depict coverage areas, shown asshaded circles, of the beam patterns generated by SBR 1010. A beamgenerated by an SBR has a coverage volume, also known as the beam's“field-of-view (FoV)”, which is a volume in three-dimensional spacewhere, during transmission, the transmitted energy density exceeds athreshold, and where, during receiving, the received energy densityexceeds a threshold. A beam's coverage area is a projection of thebeam's FoV on a surface. The FoV and coverage area may be differentduring transmit and receive, and may vary with reader or tag power, thethresholds, the distance between the SBR and the surface, and otherparameters.

Diagram 1020 depicts the coverage area of central beam 1012. Diagram1040 depicts the coverage areas of the inner beams such as 1014 and1015. Diagram 1060 depicts the coverage areas of the outer beams such as1015 and 1016. Finally, diagram 1080 depicts the total coverage area ofall the beams formed by SBR 1010. As shown in diagrams 1020-1080, beamcoverage areas may overlap. For example, inner beam 1014 may overlapwith the central beam 1012, with one or more other inner beams, and withone or more other outer beams.

Whereas SBR 1010 is depicted as being able to generate and switchbetween five beams on an axis (e.g., axis 1018), in other embodiments anSBR may generate and switch between more or fewer beams on any givenaxis. Similarly, whereas SBR 1010 is depicted as being able to generatebeams on four different axes (e.g., axes 920, 922, 924, and 926 in FIG.9A), in other embodiments an SBR may be configured to generate beams onmore or fewer axes. An individual beam's coverage area in FIG. 10 andsubsequent figures is depicted as circular for simplicity, and inactuality may be of any suitable shape, and may vary based oninteractions between the different elements that form the beam, as wellas the orientation and topology of the surface on which the coveragearea is projected. For example, a beam may have a non-circular coveragearea. As another example, a circular beam that illuminates a surfacewith a non-perpendicular angle may project an elliptical coverage areaon the surface.

FIG. 11 depicts receive sensitivity or beam power as a function of beamangle for a subset of the potential beams of the SBR of FIG. 10. Diagram1100 is similar to diagram 1000, with similarly numbered elementsbehaving similarly. Diagram 1120 depicts a view of the overlapping areasof coverage of beams 1012, 1013, 1014, 1015, and 1016, all lying alongaxis 1018. Chart 1140 plots receive sensitivity or beam power as afunction of beam angle. Each of the beams 1012-1016 is oriented at aparticular angle, with beam 1012 oriented perpendicular to the plane ofSBR 1010, beams 1013/1014 oriented at an angle θ from beam 1012, andbeams 1015/1016 oriented at an angle φ from beam 1012. These angles aremapped on the horizontal axis 1142 of chart 1140. The vertical axis 1144may represent either beam sensitivity or delivered power (or both), withsensitivity/power decreasing away from the origin. Each of the beams1012-1016 has a sensitivity/power contour depicted as an arc in chart1140 with a maximum at the beam's pointing angle. The contours definethree-dimensional surfaces for which the receive sensitivity, beampower, or both, have a constant value. If we consider just beam power,as the tag's position moves away from the beam's pointing angle, thepower decreases, reducing the beam's ability to power the tag.Alternatively, the power contour can represent the minimum power neededto power the RFID tag, and the curves in the beam's contour canrepresent the reduction in read range as the tag moves away from thebeam's pointing angle. Of course, beam sensitivity/power may vary withbeam RF frequency, tag sensitivity, desired tag operation, or myriadother RFID parameters as will be known to those skilled in the art.

FIG. 12 is a diagram depicting an RFID tag located such that it isilluminated by a subset of the beams of FIG. 1. Chart 1200 plots beampower 1202 available to power the tag from beam 1012. Horizontal axis1206 represents position and vertical axis 1204 power. A tag's abilityto extract power from an incident RF wave varies with RF frequency, formany reasons, including the fact that a tag antenna is typically moresensitive at certain frequencies than others. Line 1210 is the powerrequired by the tag at the worst-case (i.e. least sensitive) frequency,and line 1222 is the power required by the tag at the best-case (i.e.most sensitive) frequency. The tag is able to operate at all physicallocations and frequencies for which curve 1202 lies above line 1210, andis able to operate at all physical locations and the best-case frequencyfor which curve 1202 lies above line 1218. Of course, there exist acontinuum of lines between 1210 and 1218, representing the continuum ofRF frequencies between best and worst case. This continuum offrequencies is represented by tag operating profile 1219.

Diagrams 1220 and 1240 are similar to diagrams 120 and 1140, withsimilarly numbered elements behaving similarly. Diagram 1240 is similarto diagram 1200, but with tag position converted from physical positionto beam pointing angle. Tag 1222, shown with an “X” in diagram 1220 and1240, is located where multiple beams overlap, and can be powered by abeam when the beam's power contour lies above line 1218 for the tag'sbest-case operating frequency. Line 1210 is not shown in diagram 1240for reasons on clarity, but tag operating profile 1219 shows the rangeof power levels at various frequencies for which the tag is able tooperate. It is clear that some beams can power the tag at allfrequencies; some beams at some frequencies; and some beams at nofrequencies.

FIG. 13 depicts how the location of the RFID tag 1222 of FIG. 12 can bedetermined using beam power levels. FIG. 13 includes a series of charts(1310, 1320, 1330, and 1340) and area-of-coverage diagrams (1312, 1322,1332, and 1342) of the subset of potential beams of FIG. 12. Tag 1222 isrepresented as an “X” in the area-of-coverage diagrams, and tagoperating profile 1219 shows the range of power levels at variousfrequencies for which the tag is able to operate.

Consider diagrams 1310 and 1312, which depict beam 1012 interacting withtag 1222. The entire tag operating profile 1219 lies within the contourof beam 1012 so the tag is able to operate regardless of beam 1012'sfrequency. For beam 1012, the SBR can read tag 1222 at each frequency,resulting in a 100% response rate with frequency, depicted as “[100]” indiagram 1312.

Turning now to beam 1013, notice that tag operating profile 1219 exceedsbeam 1013's power for a small subset of frequencies. As a result, theSBR can read tag 1222 at a subset of frequencies, resulting in a 92%response rate with frequency, depicted as “[92]” in diagram 1322.

Turning to beam 1014, notice that tag operating profile 1219 exceedsbeam 1013's power for a larger subset of frequencies. As a result, theSBR can read tag 1222 over a smaller subset of frequencies, resulting ina 60% response rate with frequency, depicted as “[60]” in diagram 1332.

Finally, diagrams 1340 and 1342 depict how each of the five beams1012-1016 may interact with tag 1222. As described above, an SBRswitching between beams 1012, 1013, and 1014 may have response rates of100%, 92%, and 60% when interacting with tag 1222. For outer beams 1015and 1016, whose contours barely overlap the tag range marker if at all,the response rates are 2% (“[2]”) and 0% (“[0]”). If the SBR measuresthe response rate of the five overlapping beams then it can determinethe beam location of tag 1222. In this specific example, beam 1012 has a100% response rate, so tag 1222 lies somewhere within its coverage area.Beam 1013 has a larger response rate than beam 1014, so tag 1222 iscloser to beam 1013 than to beam 1014. In some embodiments, the responserates can be combined, for example using a weighted averaging orcentroid-averaging method, to improve the estimate of tag location. Thelocation of tag 1222 may be further refined by using the slopes of thebeam sensitivity responses compared to the tag operating profile.

Whereas the tag location in FIG. 12 was determined, for reasons ofbrevity and clarity, using beams lying along axis 1018, other beamsalong other axes may also be used. FIG. 14 depicts how a tag's locationmay be determined using multiple beams in 2-dimensional space. Diagram1400 depicts beam patterns similar to those in diagram 1080 of FIG. 10.For clarity, each beam in diagram 1400 is represented by a small shadedcircle that does not necessarily represent its coverage area. Tag 1422,similar to tag 1222, is represented by an “X”. Each beam has anassociated tag response rate (as described in FIG. 13) represented bythe bracketed number in the beam's circle. The response rate for a beamis related to the overlap between the beam's coverage and the locationof tag 1422. Beams that are closer to tag 1422 have higher responserates than beams that are farther from tag 1422. The location of tag1422 may be determined using the different response rates of thedifferent beams.

Although FIGS. 13 and 14 show specific response rates, these values areprovided for illustration, and may not correspond to actual responserates. Similarly, the relationships between the response rates providedby adjacent beams shown in FIGS. 13 and 14 are for illustration, and maydiffer from the actual response rates between adjacent beams.

Whereas a beam's coverage area is depicted as circular in the figuresabove, in some embodiments a beam's coverage area may be shaped. As oneexample, FIG. 15 depicts beams with elliptical shapes formed by an SBRaccording to embodiments.

Diagram 1500 in FIG. 15 is similar to FIG. 9B and depicts beamssynthesized by activating antenna elements along plane 920. As describedin FIG. 9B, RF beams 1530, 1532, and 1534 (similar to RF beams 930, 932,and 934, respectively) can be synthesized by supplyingappropriately-phase-shifted signals to the antenna elements along plane920. Because only 3 antenna elements are activated, in a line, the SBRof diagram 1500 is able to steer and shape a beam in the verticaldirection in diagram 1500 but not in the horizontal direction. Ofcourse, activating additional elements can shift and shape the beam inother directions. Diagram 1540 shows elements activated along line 922to generate a beam of shape 1542, and elements activated along line 924to generate a beam of shape 1546. Adjusting the phase of the signalsprovided to the antenna elements along lines 922, 924, and 926 allowssteering beams to different locations, as shown in diagram 1560 forbeams 1562, 1564, and 1566, respectively. As described above, a tag atlocation 1546 in diagram 1560 may produce different response rates withfrequency to an SBR steering a beam in these various directions,allowing the SBR to determine the tag's location.

All steered-beam systems generate side beams (sidelobes) in addition tothe main beam. As will be known to those skilled in the an, there existtechniques to suppress these sidelobes, such as by adding more steeringelements, digital beam shaping, shielding, and other methods, but nonecan suppress the sidelobes entirely. FIG. 16 depicts steered beams withsidelobes formed by an SBR according to embodiments. Diagram 1600,similar to FIG. 9B, depicts a head-on view to the left and a side viewto the right of an SBR generating an RF beam 1630 having a sidelobe1632, which may inventory a tag that is not positioned along the beam'smain lobe. For example, beam 1630 points upward, but sidelobe 1632,pointing downward, can erroneously inventory tag 1634. Fortunately,because sidelobes have lower power than main lobes, an SBR can typicallyuse a tag's response rate to discriminate a tag in a main beam (whichwill have a high response rate) from a tag in a sidelobe (which willhave a low response rate).

As described above, a tag's sensitivity and response rate may vary withthe frequency of the incident beam. In some embodiments, a beam'ssensitivity contour may also vary with frequency. FIG. 17 depicts howfrequency-based variations in a beams delivered power can be used todetermine tag location. Graph 1700 is similar to graphs 1310, 1320, and1330, and shows a beam having sensitivity contour 1702 at one frequencyand sensitivity contour 1704 at another frequency. Graph 1700 alsodepicts a tag operating profile 1719 similar to profile 1219. By usingbeam profiles along with the tag response rates as a function offrequency, an SBR may be able to better estimate the tag's location.

FIG. 18 depicts a process 1800 for determining tag location by countingtag reads. Process 1800 begins at operation 1802, where a tag isinventoried by an SBR. At operation 1804, the SBR sweeps throughdifferent beams and different frequencies for each beam, counting thetag response rate in operation 1806. For example, the SBR may generateand count tag reads using a first beam at a first frequency, thengenerate and count tag reads using a second beam at the first frequency,and continue this process until it has iterated through all beams. TheSBR may then repeat the process for different frequencies.Alternatively, the SBR may count tag reads using a first beam at a firstfrequency, then generate and count tag reads using the first beam at asecond frequency, and continue this process until it has iteratedthrough all frequencies. The SBR may then repeat the process fordifferent beams. Of course, the SBR may interleave these choices ofbeams and frequencies in any way imaginable, including randomly. The SBRmay act adaptively/intelligently, choosing a subset of beams andfrequencies that allow it to locate a tag as quickly and/or accuratelyas possible. The SBR may alter the polarization of its beams to reducethe impact of tag polarization when determining tag location. The SBRmay combine beam/frequency data with other information, such as a map orplanogram of a facility (e.g. the SBR may exclude known barriers such aswalls from the potential locations for a tag) or tag type (e.g. sometags are more sensitive or directional than others. Finally, atoperation 1808, the SBR uses the collected beam, frequency, read rate,and other information to determine the tag location.

A beam's ability to inventory or access a tag is based on the absolutebeam power, beam power contour, beam pointing direction, beam frequency,distance to the tag, beam and tag polarization, type of tag, type of tagoperation (e.g. inventory versus writing), and other parameters. A tagthat lies within the SBR's tag inventory range has sufficient power toreceive and respond to SBR commands, whereas a tag that lies outside theSBR's tag inventory range does not have sufficient power to receive andrespond to SBR commands.

FIG. 19 depicts an SBR's tag inventory range according to embodiments.Diagrams 1920 and 1940 show side and head-on views of an SBR 1900attempting to inventory tags 1904 and 1906 using a beam 1902. Each ofthe tags 1904/1906 has an associated sensitivity contour, labeled as1908 and 1910, respectively. SBR 1900 can inventory tag 1904 using beam1902, because beam 1902's power contour overlaps tag sensitivity contour1908. SBR 1900 cannot inventory tag 1906 using beam 1902, because beam1902 does not overlap tag 1906's sensitivity contour 1910.

Suppose that there was a way to deliver additional power to tag 1906 andthereby increase the size of its sensitivity contour 1910. Asufficiently large increase would allow beam 1902 to inventory tag 1906.Of course, one way to increase the size of tag 1906's sensitivitycontour is to add a battery to tag 1906. However, it may be desirable tofind a way to do so without artificial means such as a battery.

FIG. 20 depicts how a tag's sensitivity contour can be increased byusing a cooperating SBR to deliver additional RF power, according toembodiments. FIG. 20, similar to FIG. 19, depicts side and head-on views2020 and 2040, respectively, of SBR 1900 attempting to inventory tags1904 and 1906. FIG. 20 also includes another SBR 2000 positioned todeliver additional RF power to tag 1910 using beam 2002. In FIG. 20 asdepicted, neither SBR 1900 nor SBR 2000 can inventory tag 1906 on itsown, because beams 1902 and 2002 lie outside sensitivity contour 1910 oftag 1906. However, if SBR 1900 and SBR 2000 deliver RF power at the sametime, then the effective sensitivity contour of tag 1906 increases fromperimeter 1910 to perimeter 2010, allowing both SBR's to inventory tag1906. In practice, there are many ways for SBR 1900 and SBR 2000 tocooperate. SBR 1900 can issue inventory commands while SBR 2000 sendsraw RF power (a continuous or minimally modulated wave). Or SBR 1900 cansend raw RF power while SBR 2000 issues inventory commands. Or both SBR1900 and SBR 2000 can issue synchronized inventory commands. Regardlessof the means, two SBRs can deliver more power than one, and if the twoSBRs are synchronized to cooperatively provide power than tag 1906,which was invisible to either SBR singly, can become visible to one orboth of them.

In some embodiments, the two SBRs may use the same RF frequency duringsuch cooperative powering, particularly in cases where the SBRs sendsynchronized commands. In other embodiments, particularly those in whichone SBR provides commands and the other raw RF power, the SBRs may usesignificantly different frequencies to avoid the RF frequenciesgenerating beat notes that confuse the tag's demodulator (such asdemodulator 442 in FIG. 4). Of course, the SBRs may optimize theirchoice of frequency based on beam or tag polarization, tag type, thetag's ability to reject interference such as beat notes, and otherparameters. The SBR's may also adjust their relative or absolutedelivered power to improve the cooperative powering. In some embodimentsone of the SBRs need not be an SBR at all, but could instead be a fixed(i.e. not steered beam) reader, a handheld read, a shelf reader, adock-door reader, or any other RFID reader as will be known to thoseskilled in the art. In some embodiments a controller and/or one or bothof the SBRs can adjust the power and/or frequency of the generated RFsignal(s) to increase the inventorying range of the RFID tag. In someembodiments the adjustment may include sweeping over a range of powerand/or frequency values; in other embodiments the adjustment may beadaptive and based on environmental conditions (for example, RF noise orthe dielectric properties of items to which tags are attached), receivedtag replies, tag performance characteristics (for example, sensitivity,interference rejection or ability to harvest power from another RFsource), and/or tag population size. In some embodiments, one of theSBRs will point to the target location and another SBR may steer itsbeam to multiple locations in the vicinity of the target location toimprove the cooperative powering.

Of course, the benefits of cooperative tag powering need not be limitedto a pair of SBRs. FIG. 21 depicts how multiple SBRs can cooperate toinventory a tag population according to embodiments. Diagram 2100depicts four SBRs 2102, 2104, 2106, and 2108 arranged to inventory a tagpopulation 2110. In diagram 2100, SBR 2102 is configured to communicatewith tags 2110 using beam 2112, while SBRs 2104-2108 are configured toprovide power to tags 2110 using beams 2114, 2116, and 2118,respectively. In diagram 2150, SBR 2108 is instead configured toinventory tags 2110 using beam 2118, while SBRs 2102-2106 provideadditional power. Of course, any of the SBRs can do the inventoryingwhile the others deliver power, or two can inventory (synchronously ornot) while the other two deliver power, or any other possibility. Ofcourse, the number of SBRs is arbitrary—four are shown but more or lesscan be used in actual practice.

In some embodiments a controller, such as controller 2120 in diagrams2100 and 2150, can perform SBR coordination to cooperatively inventoryand power tags using SBRs 2102, 2104, 2106, and 2108. In otherembodiments controller 2120 can be embedded within one or more of theSBRs. In yet other embodiments the SBRs can form a peer-to-peercommunication network and synchronize with each other.

In general, an SBR synthesizes an RF beam to point at locations (e.g.beam areas-of-coverage shown in FIG. 10), for durations, and at timesaccording to a scanning pattern, which may be predetermined or dynamic.A pointing location can be identified by the one or more SBRs as a beamindicator (such as a numeric indicator), a location on the floor of afacility in which the SBRs are located, a set of Cartesian or polarcoordinates, or any other suitable location identifier.

In some embodiments, the scan pattern is a sequence of target locationsand an SBR may synthesize beams to point at the different targetlocations based on a timer, a trigger signal generated by the SBR or acontroller, and/or communications from one or more other SBRs. In someembodiments, the scan pattern is at least one target location and atleast one corresponding target-location time, defined as the time atwhich two different SBRs point to the target location. Thetarget-location time may be absolute (for example, 4:00 pm) or relative(for example, ten milliseconds after a trigger or timer signal orcommunication). An SBR may store the scan pattern in memory, receive ascan pattern from a controller (e.g., controller 2120, another SBR, anetwork device, or some other controlling entity), generate a scanpattern using information received from a controller or other SBRs,generate a scan pattern randomly, generate a scan pattern to optimizetag cooperative powering, or generate a scan pattern based on any othersuitable parameter(s). In some embodiments, an SBR's scan pattern may beoverridden temporarily (or permanently) by a controller or another SBR.

The assignment of roles (inventorying or powering) to different SBRs maydepend on history (i.e. whether an SBR was most recently an inventoryingSBR or a powering SBR), the SBR's location, the number of tags the SBRhas inventoried recently or historically, the number of tags all or asubset of the SBRs have inventoried, or any other number or type ofsuitable parameters. In some embodiments the SBR role assignments may bepreset—for example, a particular SBR may be assigned to inventory for aperiod of time, provide power for another period of time, and thenrepeat. In other embodiments SBR role assignments may be dynamic oradaptive, even during an ongoing communication with a tag. For example,one reader may begin and then interrupt a dialog with a tag. Anotherreader may then continue the interrupted dialog with the tag while theoriginal reader provides power or performs some other task.

In some embodiments, SBRs may be configured to cooperatively power tagsonly in certain circumstances. For example, if one SBR is using one ofits outer beams to inventory tags then other SBR may cooperatively powerthe tags, knowing that cooperative powering is particularly effectivewhen used with outer SBR beams. As another example, if a tag is movingtoward the periphery of an SBR's field-of-view (and therefore into itsouter beam coverage), other SBR may cooperate to provide additionalpower. As yet another example, if the number of inventoried tags is lessthan an expected number, other SBRs may provide additional power toboost tag sensitivity and thereby allow more tags to be inventoried. Asyet another example, if a tag indicates that it has insufficient powerto perform an operation, such as writing data to memory, other SBRs mayprovide additional power so the tag can perform the operation.

As described above, SBRs may be configured to receive and/or exchangeinformation about target locations, scan patterns, scan timing, beamconfiguration, tags, cooperative powering, and roles. FIG. 22 depicts avariety of ways in which SBRs can receive and/or exchange suchinformation. Diagram 2200 depicts a first configuration in which amaster SBR 2202 coordinates the operations of two slave SBRs 2204 and2206. Diagram 2240 depicts a second configuration in which three SBRs,2042, 2044, and 2046 coordinate operation via peer-to-peercommunications. Diagram 2280 depicts a third configuration in which acentralized controller 2282 coordinates the operations of three SBRs2284, 2286, and 2288. Of course, multiple variants on these themes arepossible including using more or less SBRs; mixing the configuration(for example, a controller coordinating peer-to-peer communications);using multiple controllers, and endless other combinations as will beobvious to those of ordinary skill in the art. Communication betweenSBRs and controllers (if present) can be implemented using a wiredconnection (e.g., Ethernet, parallel, serial, or other suitable wiredprotocol), a wireless connection (e.g. WiFi, cellular. Bluetooth, orother suitable wireless protocol), a point-to-point protocol, a packetor address-based protocol, or any other suitable connection type orprotocol.

FIG. 23 depicts a process 2300 for coordinating SBR operation accordingto embodiments. Process 2300 begins at operation 2302, where a first SBR(e.g., SBR 2000) synthesizes a beam (e.g., beam 2002) to point at afirst location. At operation 2304, the first SBR transmits a minimallymodulated (power) signal. At operation 2306, a second SBR (e.g., SBR1900) synthesizes a beam (e.g., beam 1902) to also point at the firstlocation. At operation 2308, the second SBR inventories one or more tagsat the first location while the first SBR continues to transmit power.At optional operation 2310, first and second SBRs repeat the aboveoperations while pointing at a second location, for example as describedabove in FIG. 21. At optional operation 2312 the first and second SBRsmay switch roles, with the first SBR inventorying tags while the secondSBR transmits power. Of course, in other embodiments more than two SBRsmay participate in process 2300.

The SBR functionality described above may be combined with tagpopulation management techniques to improve inventory performance anddetect tag movement. One such tag population management technique is atag refresh, also described in copending U.S. patent application Ser.No. 13/271,937 entitled “Broadcast Refresh of RFID Tag Persistence,”which is hereby incorporated by reference in its entirety. Many RFIDsystems expect a reader, such as an SBR, to inventory all the tags in apopulation. Typically, the reader inventories a first tag, instructs thetag not to reply for a period of time (a persistence time), and thenproceeds to inventory a second tag. In typical embodiments the readerinstructs the first tag, either explicitly or implicitly, to set a flag(an inventoried flag, which is usually a semi-persistent bit valuestored on the tag) from a default or “unset” value to a “set” value,thereby indicating that the tag has been inventoried. After some time(the flag persistence time), the flag typically decays back to thedefault value. In an ideal world, the reader would continue the processuntil it has inventoried all of the tags. Unfortunately, if the readerhas not finished inventorying all of the tags within the persistencetime then the first tag will forget that it has already been inventoried(i.e., its inventoried flag will decay to its default value) and presentitself to be inventoried again. Subsequently, a second tag may exhibitthe same behavior, followed by other tags. If the tag population islarge then the reader may spend so much time re-inventoryingalready-inventoried tags that it cannot spend enough time searching forthe hard-to-inventory tags. As a result, some of the tags in thepopulation may not be inventoried at all.

FIG. 24 is a diagram 2400 showing the effects of a tag refresh on aphysical flag parameter as a function of time, according to embodiments.A flag parameter may include one or more of voltage, current, charge,and flux associated with a tag flag. In some embodiments, if the valueof a flag parameter is below a threshold 2402 then the flag isconsidered to hold a first value (for example, the value “A”), whereasif the value of the parameter is above the threshold 2402 then the flagis considered to hold a second value (e.g., the value “B”). During aninventory operation 2406 the flag parameter may be switched. Althoughthe inventory operation 2406 in FIG. 24 asserts the tag flag value from“A” to “B”, in other embodiments an inventory operation may assert thetag flag value from “B” to “A”. The amount by which the flag physicalparameter is adjusted during a switching operation may be static (e.g.,always increased/decreased by a preset amount) or dynamic (e.g., theamount of the increase/decrease varies according to any number ofparameters), as long as the adjustment amount is sufficient to changethe flag value.

After inventory operation 2406 switches the flag value from A to B, theflag physical parameter will begin decaying, as indicated by curve 2404.At some point in time 2408 the flag physical parameter will have decayedbelow threshold 2402, switching the flag value from B back to A. Thedifference between time 2408 and time 2406 is the flag's persistencetime, and is how long the flag holds the value B. The rate at which thephysical parameter decays may be a function of one or more tag and/orenvironmental conditions, such as tag design or temperature.

If a tag is capable of executing a flag refresh, and a reader transmitsa refresh command 2412 to the tag before time 2408 (i.e. before thephysical parameter decays below threshold 2402), then the refreshcommand 2412 adjusts (or instructs the tag to adjust) the physicalparameter to increase the flag persistence time. As above, the amount ofthe adjustment may be static or dynamic, as long as the post-refreshvalue is different from the value before the refresh command. Bytransmitting successive refresh commands (e.g., refresh command 2413followed by refresh command 2414) the resulting decay curve 2410 can beadjusted such that the effective flag persistence time (i.e., the timebefore which curve 2410 drops below the threshold 2402) can be extendedas far beyond the normal flag persistence time as desired. A reader maytransmit the refresh command 2412 to individual tags or may broadcastthe refresh command 2412 to multiple tags simultaneously. The term“broadcast” as used herein implies that the command is directed tomultiple tags rather than to a singulated tag.

FIG. 25 is a conceptual diagram showing an illustrative inventoryingprocess 2500 without refresh according to embodiments. The inventoryingprocess 2500 begins at the left and proceeds to the right along thehorizontal TIME axis. At time 2504, all tags have inventoried flagvalues of “A” and the reader 2502 begins inventorying “A” tags. At time2506, three tags have been inventoried by reader 2502 and theirinventoried flag values have been switched to “B”, but three tags havenot yet been inventoried by reader 2502 and their flag values remain as“A”. After time 2506, the reader inventories an additional two tags andswitches their flags to “B”. However, the flag value of one of thepreviously-inventoried tags 2514, possibly the first tag inventoried,exceeds its persistence time and decays back to “A” after time 2506. Asa result, at time 2508, while five of the six tags have beeninventoried, reader 2502 still has two tags in the “A” state that itneeds to inventory. After time 2508, the reader 2502 manages tore-inventory tag 2514 and switch its flag value back to “B”, but in themeantime previously-inventoried tags 2516 and 2518 have decayed back to“A”. Thus, at time 2510, although reader 2502 has inventoried five ofthe six tags, it has not yet inventoried the last tag 2512, and it maynot be able to devote sufficient time to inventory tag 2512 if thepreviously inventoried tags keep decaying.

FIG. 26 is a conceptual diagram showing an illustrative inventoryingprocess 2600 with refresh according to embodiments. As with inventoryingprocess 2500 described above in relation to FIG. 25, the inventoryingprocess 2600 begins at time 2604 (similar to time 2504 in FIG. 25) andproceeds to time 2606 with three tags inventoried, similar to time 2506in FIG. 25. Differently from FIG. 25, after time 2606 reader 2602transmits or broadcasts a refresh command, and tag 2614's flag valuedoes not decay back to “A” but instead stays in “B” (differently fromtag 2514's flag value in FIG. 25, which does decay). As a result, attime 2608, live of the six tags have been inventoried, and none havedecayed back to the “A” state. At time 2608, reader 2602 is able todevote sufficient time to inventory the last tag 2612, and a refreshcommand after time 2608 prevents tags 2616 and 2618 from decaying backto the “A” state at time 2610 (compare to tags 2516 and 2518 in FIG. 25,which do decay back to “A”). Thus, the commanded refresh functionalityallows reader 2602 to spend more time searching for and inventoryinguninventoried tags instead of inventorying previously inventoried tags.

The refresh functionality described above need not be used only whileactively inventorying tags. For example, a reader may broadcast refreshsignals to already-inventoried tags while waiting for other tags toenter its field-of-view. The refresh functionality may also be used toassist in managing different populations of tags. For example, a readermay inventory a first population of tags in a first session and usebroadcast refresh signals to maintain the values of the tag flagsassociated with the first session. The reader may then inventory asecond population of tags in a second session different from the first,while continuing to broadcast refresh signals to maintain the flagvalues of tags in the first population. The sessions concept isdescribed in the UHF Gen2 Specification.

As suggested above, this refresh functionality may assist in determiningthe direction of a tag's movement in an environment that containsmultiple other tags. FIG. 27 is a conceptual diagram 2700 showing sideviews of an SBR system at different stages of a tag motion trackingprocess according to embodiments.

In diagram 2700, an SBR 2702 is configured to generate outer RF beams2712, 2714, 2718, and 2720, and a central RF beam 2716. SBR 2702typically scans among its different beams 2712-2720 (and also betweenother beams not depicted in FIG. 27) continuously, to look for tagswithin its coverage area (e.g., as depicted in diagram 1080).

At stage 2710 of the tag-motion tracking process, SBR 2702 firstinventories stationary tags S1-S4 in container 2722 and stationary tagsS5-S6 in container 2724 in a first session. SBR 2702 then broadcasts arefresh signal to the stationary tags to keep them quiet in the firstsession. In some embodiments, these stationary tags may also beinventoried in a second session. In some embodiments the inventoriedflags in the second session do not decay while the tags receive power.

At subsequent stage 2730, a container 2732 with tags of interest (TOIs)2734, 2736, and 2738 moves rightward into SBR 2702's coverage area. Asthey enter the coverage area, SBR 2702 inventories these TOIs with outerRF beams 2712 and 2714, using the second session for the inventorying.Upon observing these new, previously unseen TOIs, SBR 2702 begins atracking process that includes using an alternative session that isdifferent from the second session (such as the first session, althoughany session other than the second may be used).

An inventorying process (such as described in the UHF Gen2Specification) involves a series of steps involving the exchange ofinformation between a reader (such as SBR 2702) and a tag. The processcauses state changes in the reader and/or tag—for example, the readermay request an identifier from the tag, the tag may reply with itsidentifier, the reader may acknowledge receipt of the identifier, andthe tag may then assert an inventoried flag in response to theacknowledgement. In some embodiments a reader may wish to inventory atag without causing the tag to assert its inventoried flag. The readermay accomplish non-acknowledgment by beginning the inventorying processas described above, but either not acknowledge receipt of thetag-provided identifier or transmitting a non-acknowledgement command tothe tag. The NAK command in the UHF Gen2 Specification is one suchnon-acknowledgment command.

At stage 2740, SBR 2702 inventories tags with central beam 2716 usingthe alternative session. Because TOIs 2734-2738 have not yet beeninventoried in the alternative session, their inventoried flags shouldbe in the A state (i.e. denoting not inventoried), and they shouldrespond to SBR 2702. When SBR 2702 receives an identifier from one ofthese TOIs 2734-2738, which it previously determined to be new andof-interest, SBR 2702 transmits a non-acknowledgement command to theTOI, thereby causing the TOI not to change the state of its inventoriedtag and thus facilitating subsequent reinventorying. Moreover, becauseSBR 2702 previously inventoried the TOI with its outer RF beams and isnow inventorying the TOI with its central RF beam, it may infer that theTOI has moved from the coverage area of outer RF beams 2712/2714 to thecoverage area of central RF beam 2716.

As mentioned above, an RFID reader (such as SBR 2702) mayunintentionally miss (i.e. not inventory) a tag (such as one of TOIs2734-2738) in one of its beams. To compensate for such misses, SBR 2702may associate TOIs that it believes to be traveling together, such asTOIs 2734-2738, in a set. SBR 2702 may form this belief from the factthat it inventoried TOIs 2734-2738 at the same time, from tracking TOIs2734-2738 moving together for a period of time, from informationprovided to it about TOIs 2734-2738, from similar TOI identifiers, orfrom other information or sources or characteristic of the TOIs. Byassuming the association among the TOIs, even if SBR 2702 misses one ofthe TOIs in a beam, such as in beam 2740, it may still assume that themissed TOI is moving with the others in the set.

At stage 2750, SBR 2702 inventories TOIs 2734-2738 with outer RF beam2720. Similar to stage 2740, SBR 2702 may inventory the TOIs in thealternative session and, after receiving a TOI identifier anddetermining that the TOI was previously inventoried, may transmit anon-acknowledgement command to the TOI.

In the example of FIG. 27, by first inventorying the tag in the secondsession, with all static tags already in the B state, SBR 2702 canquickly identify new TOIs. By using non-acknowledgment commands in thealternative session, SBR 2702 can continue to observe the TOIs in a seaof static tags. By tracking a TOI across beams 1212, 2714, 2716, and2720, SBR 2702 and may infer that a TOI is moving.

Whereas the example in FIG. 27 depicts tracking TOIs moving linearly inone direction (rightward), SBR 2702 may be configured to track TOIsmoving in other directions, in linear or nonlinear paths. For example,SBR 2702 can be configured to track TOIs that enter its coverage areaalong one axis and leave along another (i.e., TOIs that change movementdirection within the SBR coverage area). Similarly, SBR 2702 can beconfigured to track TOIs that move in a curved path.

In some embodiments, an SBR may use tag refresh commands as describedabove to assist in tag tracking. For example, SBR 2702 may use refreshcommands to maintain the inventoried flags of the static tags incontainers 2722 and 2724. FIG. 28 shows a timing diagram 2800 for a tagtracking process with tag refresh commands according to embodiments.Timing diagram 2800 displays the values of the session one (S1) and two(S2) flags a TOI 2802 and two static tags 2804 and 2806. In timingdiagram 2800, a flag is asserted when its value is high and not assertedwhen its value is low. The horizontal axis of the timing diagram 2800represents time, with events to the left preceding events to the right.

At initial time 2810, an SBR inventories static tags 2804 and 2806 inboth S1 and S2 sessions and instructs them to assert their S1 and S2flags. Whereas timing diagram 2800 depicts these tags being inventoriedin both sessions at the same time, in typical embodiments the tags arefirst inventoried in one session and then in the other session.

At subsequent time 2812, the SBR transmits a refresh command to statictags 2804 and 2806 as described above. Without the refresh command attime 2812, the S1 flags of static tags 2804/2806 would decay (asdepicted by the dotted curves) shortly after time 2812. In the depictedembodiment, the S2 flags of the static tags 2804/2806 do not decay,because a tag's S2 flag value persists when the tag is powered (asdescribed in the Gen2 Specification).

At time 2814, the SBR inventories TOI 2802 in session S2 and causes itsS2 session flag to be asserted as a result of being inventoried.

At time 2816 the SBR transmits another refresh command to static tags2804/2806 to maintain their S1 flag values, which would otherwise decay.

At time 2818, the SBR inventories TOI 2802 in session S1. Afterreceiving an identifier from TOI 2802, the SBR transmits anon-acknowledgement command (such as a Gen2 NAK command), causing TOI2802's S1 session flag to remain deasserted.

At times 2820 and 2822 the SBR transmits refresh commands to maintainthe S1 session flag values of static tags 2804 and 2806. At time 2824,the SBR again inventories TOI 2802 in session S1 and terminates theinventorying process by transmitting a NAK command, leaving TOI 2802'sS1 session flag of deasserted.

By first using session S2 to find TOI 2802 among static tags whose S2session flags are held asserted by being powered, and then using sessionS1 and NAKs to read TOT 2802 multiple times among static tags whose S1session flags are refreshed, the SBR is able to inventory TOI 2802, inmultiple beams as described above, and track its movement. Of course,the above session-flag choices are arbitrary—session flags S1 and S2could be swapped, or session flag S3 could be used instead of sessionflag S1, or session flag S3 could be used instead of session flag S2, orthe tags could have customer session flags with different names andattributes.

FIG. 29 is a flowchart of a tag tracking process 2900, as may beperformed by an SBR such as SBR 2702, according to embodiments.

In step 2902 the SBR determines whether to inventory tags or transmit aflag refresh command. The SBR may determine whether to transmit arefresh command based on whether it has recently inventoried any TOIs,the time since the last refresh command, the number of static tags, orany other suitable condition or combination of conditions.

If the SBR chooses to send a refresh command then it transmits a refreshcommand in step 2906, as described above in relation to step 2706 inFIG. 27. If the SBR chooses to inventory tags then it may receive anidentifier from a static tag or a TO. The SBR may determine whether atag is static or a TOI based on any of the above-described criteria,such as whether the SBR has observed the tag previously, whether the tagis moving, whether the tag has an identifier of interest, etc. The SBRmay use multiple of its RF beams to inventory tags, and may use one ormore sessions. Unless the SBR observes a TOI it returns to step 2902.

If the SBR finds a potential TOI then, in step 2908, it determines ifthe TOI was previously observed. If confirmed then the SBR may evaluatea TO parameter (e.g., whether the TOI is moving, which direction it camefrom/is going, speed, path, etc.) in step 2910. If the TOI is notconfirmed then the SBR returns to step 2902 without evaluating a TOIparameter. Subsequently, the reader returns to step 2902.

In some embodiments, the SBR prioritizes the order in which it performsthe tasks in step 2902 either in a fixed sequence or dynamically, basedon information received from tags or from external sources.

Whereas the above tag-tracking process uses session S1, which decaysover time, in other embodiments the process may use a different sessionthat does not decay while the tag is energized, such as S3. In theseembodiments refresh step 2906 may be omitted.

The steps described in processes 1800, 2300, and 2900 are forillustration purposes only. RFID tag management using SBRs may beperformed employing additional or fewer steps and in different ordersusing the principles described herein. Of course the order of the stepsmay be modified, some steps eliminated, or other steps added accordingto other embodiments.

Whereas in the above description the RF beams for transmitting andreceiving are synthesized by an SBR, in some embodiments one or more ofthe beams, in either or both of the transmit and receivefunctionalities, may be generated without the use of a synthesized-beamantenna. For example, the transmit beams may be generated by asynthesized-beam antenna but the receive beam may employ a staticantenna such a a patch, monopole, dipole, etc. As another example, thesynthesized beams may be replaced by multiple static antennas coupled toone or more readers.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams and/orexamples. Insofar as such block diagrams and/or examples contain one ormore functions and/or aspects, it will be understood by those within theart that each function and/or aspect within such block diagrams orexamples may be implemented individually and/or collectively, by a widerange of hardware, software, firmware, or virtually any combinationthereof. Those skilled in the art will recognize that some aspects ofthe embodiments disclosed herein, in whole or in part, may beequivalently implemented employing integrated circuits, as one or morecomputer programs running on one or more computers (e.g., as one or moreprograms running on one or more computer systems), as one or moreprograms running on one or more processors (e.g. as one or more programsrunning on one or more microprocessors), as firmware, or as virtuallyany combination thereof, and that designing the circuitry and/or writingthe code for the software and or firmware would be well within the skillof one of skill in the art in light of this disclosure.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isto be understood that this disclosure is not limited to particularmethods, configurations, antennas, transmission lines, and the like,which can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations).

Furthermore, in those instances where a convention analogous to “atleast one of A, B, and C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g., “a system having at least one of A, B, and C”would include but not be limited to systems that have A alone, B alone,C alone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or “B” or “A and B.”

As will be understood by one skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember. Thus, for example, a group having 1-3 cells refers to groupshaving 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers togroups having 1, 2, 3, 4, or 5 cells, and so forth.

We claim:
 1. A method for inventorying a Radio Frequency Identification(RFID) tag comprising: communicating, from a controller, a targetlocation to a first synthesized-beam RFID reader and to a secondsynthesized-beam RFID reader, steering, responsive to the communicatedtarget location, a first beam from the first synthesized-beam reader tothe target location and a second beam from the second synthesized-beamreader to the target location; transmitting synchronized inventorycommands from the first and second synthesized-beam readers; receiving,responsive to the synchronized inventory commands, a tag reply at atleast one of the first and second synthesized-beam readers; andtransmitting, from at least one of the first and second synthesized-beamreaders, an acknowledgment signal responsive to the tag reply.
 2. Themethod of claim 1, further comprising transmitting the acknowledgementsignal from the first and second synthesized-beam readers.
 3. The methodof claim 1, wherein the target location includes at least one of: a beamindicator; a floor location; a Cartesian coordinate; and a polarcoordinate.
 4. The method of claim 1, further comprising adjusting atleast one of a power and a frequency of at least one of the synchronizedinventory commands.
 5. The method of claim 4, wherein the adjustingcomprises sweeping over a range of values.
 6. The method of claim 1,further comprising steering the second beam to a plurality of locationsin a vicinity of the target location.
 7. The method of claim 1, furthercomprising synthesizing the second beam in response to at least one of:determining that the first beam is an outer beam; and determining thatthe tag is moving toward a periphery of a field-of-view of the firstsynthesized-beam reader.
 8. A method for inventorying a Radio FrequencyIdentification (RFID) tag comprising: communicating, from a firstsynthesized-beam RFID reader, a target location to a secondsynthesized-beam RFID reader; steering a first beam from the firstsynthesized-beam reader to the target location; steering, responsive tothe communicated target location, a second beam from the secondsynthesized-beam reader to the target location; transmittingsynchronized inventory commands from the first and secondsynthesized-beam readers; receiving, responsive to the synchronizedinventory commands, a tag reply at at least one of the first and secondsynthesized-beam readers; and transmitting, from at least one of thefirst and second synthesized-beam readers, an acknowledgment signalresponsive to the tag reply.
 9. The method of claim 8, furthercomprising transmitting the acknowledgement signal from the first andsecond synthesized-beam readers.
 10. The method of claim 8, wherein thetarget location includes at least one of: a beam indicator; a floorlocation; a Cartesian coordinate; and a polar coordinate.
 11. The methodof claim 8, further comprising adjusting at least one of a power and afrequency of at least one of the synchronized inventory commands. 12.The method of claim 11, wherein the adjusting comprises sweeping over arange of values.
 13. The method of claim 8, further comprising steeringthe second beam to a plurality of locations in a vicinity of the targetlocation.
 14. The method of claim 8, further comprising synthesizing thesecond beam in response to at least one of: determining that the firstbeam is an outer beam; and determining that the tag is moving toward aperiphery of a field-of-view of the first synthesized-beam reader.
 15. Amethod for inventorying a Radio Frequency Identification (RFID) tagcomprising: storing at least one target location and at least onecorresponding target-location time in first and second synthesized-beamRFID readers; steering, at the target-location time, a first beam fromthe first synthesized-beam reader to the target location and a secondbeam from the second synthesized-beam reader to the target location;transmitting synchronized inventory commands from the first and secondsynthesized-beam readers; receiving, responsive to the synchronizedinventory commands, a tag reply at at least one of the first and secondsynthesized-beam readers; and transmitting, from at least one of thefirst and second synthesized-beam readers, an acknowledgment signalresponsive to the tag reply.
 16. The method of claim 15, furthercomprising transmitting the acknowledgement signal from the first andsecond synthesized-beam readers.
 17. The method of claim 15, wherein thetarget location includes at least one of: a beam indicator; a floorlocation; a Cartesian coordinate; and a polar coordinate.
 18. The methodof claim 15, further comprising adjusting at least one of a power and afrequency of at least one of the synchronized inventory commands. 19.The method of claim 18, wherein the adjusting comprises at least one of:sweeping over a range of values; and adaptive adjustment based on one ormore of an environmental condition, a received tag reply, a tagperformance characteristic, and a tag population size.
 20. The method ofclaim 15, further comprising coordinating the target-location timebetween the first and second synthesized-beam readers by at least oneof: a timer; a trigger signal; and a communication from one of the firstand second synthesized-beam readers to the other of the first and secondsynthesized-beam readers.
 21. The method of claim 15, further comprisingsteering the second beam to a plurality of locations in a vicinity ofthe target location at the target-location time.